What’s a 5G Transport Service
The mobile communication network’s services are collectively implemented using functions spread across the RAN, xHaul, and mobile core domains. These functions seamlessly interact to implement the mobile service through connectivity provided by the xHaul network. In the early generations of mobile communications, a simple Layer 2 (L2) or circuit switched network was sufficient for backhauling mobile traffic. But in the current and future generations of mobile communications network (MCN), the xHaul networks require the flexible, scalable, cost-effective, and operationally simple solutions provided through IP- and MPLS-based networks. For these xHaul networks to provide the connectivity expected of them, transport network services, or simply transport services, are implemented as an overlay using virtual private networks (VPNs). These transport services are typically a subset of an end-to-end mobile service and have a vital role in the realization of services provided by the MCN. This chapter will explain the various technologies and methods used to implement the transport services for enabling a 5G network.
What’s a 5G Transport Service?
It should be well understood by now that the components that make up a mobile service for an end consumer reside in multiple network domains. The RAN domain contains the radio-related equipment, where a mobile subscriber connects to the mobile provider’s network. This equipment consists of antennas and baseband processing units that, by virtue of RAN decomposition, can be spread across multiple geographical locations (that is, the cell site, the far edge, and the edge DCs). The RAN devices must communicate with the 5GC functions, which are also likely to be spread over multiple data centers (DCs) through the introduction of CUPS. The 5GC provides subscriber authentication, registration, and connectivity to the data network (DN) and other required functions to enable mobile services for the end subscriber.
These mobile services would not be possible without a robust transport infrastructure, which in turn offers a transport service, enabling connectivity between mobility functions in different domains as well as within the same domain. Examples of interdomain transport services include connectivity between the centralized unit (CU) that is part of the RAN, and the user plane function (UPF) that is part of the 5GC. Intradomain service examples may include connectivity between the radio unit (RU) and distributed unit (DU) and between the DU and CU—all of which are in the RAN domain.
While the forwarding mechanism might vary with the network domain (for example, Segment Routing in xHaul, VXLAN in the DC), the overlay services almost always require some degree of isolation to allow multiple traffic types to be transported independently over the same underlying infrastructure. This traffic separation is provided by the use of a VPN that can be established entirely within the xHaul domain (for example, Xn for 5G and X2 for 4G interfaces between cell sites) or extended into different DCs in a 5G network with decomposed RAN. The next section elaborates on the type of VPN services used in the transport network to enable end-to-end mobile services.